Radiation detector



May 10, 1960 H. N. WILSON ETAL 2,936,401

RADIATION DETECTOR Original Filed Oct. 17. 1955 2 Sheets-Sheet 1 vvvv. II I/Vv v v vI I I I/VVV I I I I I I I INVENTORS. Floyd M. Glass and BYHuberf N. Wilson 4- W AT TORNE Y May 10, 1960 H. N. WILSON ET AL2,936,401

RADIATION DETECTOR Original Filed Oct. 17, 1955 2 Sheets-Sheet 2 HSVINVENTORS Floyd M. Glass and Huber) N. Wilson fiM /Q. W

ATTORNEY United States Patent RADIATION DETECTOR Hubert N. Wilson, OakRidge, and Floyd M. Glass, Morris, Tenn., assignors to the United Statesof Americaas represented by the United States Atomic Energy CommissionOriginal application 0ctober 17, 1955, Serial No. 541,104,

-1110W Patent No. 2,874,305, dated December 18, 1956. Divided and thisapplication October 29., 1958, Serial No. 770,568 i 4 Claims. (Cl.315-59 This invention relates to radiation detecting and measuringdevices, and more particularly to a compact self resetting backgroundmonitoring device and is a division of our prior co-pending applicationS.N. 541,104, filed October 17, 1955, which matured into U.S. Patent2,874,305.

One of the many problems associated with the handling of radioactivesubstances is that of determining the quantity of radiation (dose) towhich operating personnel are exposed. Certain tolerance doses have beenestablishedfor individuals beyond which serious consequences may result,so it is desirable that personnel working with radioactive substances orpresent in the area where radiation may exist know what portion of thistolerance dose they have received at any given time, and that they bewarned when the tolerance dose has been reached or exceeded.

While instruments which will integrate the radiation and provide anindication of thetotal radiation received, as well as the instantaneousmagnitude of the radiation, are commercially available, theseinstruments generally have the disadvantage that leakage currents arehigh by reason of the many leakage paths resulting from the large numberof components which are connected in parallel in the circuits of theseinstruments and the lengths of' leads necessary to couple them. Otherfactors contributing to leakage include the failure to enclose andhermetically seal the components of the system, thereby exposing them tothe :harmful effects of moisture and As a consequence, it is necessaryto compensate for the resulting leakage through the provision of largercapacitors, larger radiation detectors, and other com ponents which arethemselves subject to greater leakage and which serveto impose a furtherlimitation on the system.

Applicants with a knowledge of these problems of the prior art have foran object of their invention the provision of a radiation detector whichwill automatically reset itself after being subjected to a predeterminedquantity of radiation.

Applicants have as another object of their invention theflp'rovision ofa radiation detector wherein the grid of the'electrometer tube isdirectly connected to an electrode of the condenser thereby eliminatinglengths of connecting cable and such high impedance elements as relaysand pushbuttons for efiecting operation.

Applicants have as a further object of their invention the provision ofa radiation detecting system wherein the principal components areenclosed and hermetically sealed to insure low leakage.

Applicants have as a further object of their invention the provision ofa radiation detecting device wherein the'leakage paths are reduced to aminimum by employing only a single insulator to support the collectorelectrode and condenser plate.

7 Applicants have as a still further object of their invenice of thecondenser also function as a guard ring of the radiation detector.

Applicants have as a still further object of their inventiontheprovision of a radiation detecting devicewherein the same elementperforms the dual function of a condenser and a guard ring by employinga capacitative coupling between the circuitry of the system and anotherelectrode of the detector to periodically alter the potential thereofand reset the system.

Applicants have as a still further object of their invention theprovision of a radiation detecting device wherein the primaryaccelerating voltage for the radiation detector is placed on the outercoating rather than on the center collector electrode in order to makeion collection independent of the potential of the collector.

Other objects and advantages of our invention will appear from thefollowing specification and accompanying drawings, and the novelfeatures thereof will be means is provided for detecting ionizingradiations, a

circuit for charging the detecting element is employed,

and an arrangement to initiate a signal when a preset quantity ofradiation has been detected, and means responsive to the operation ofthe circuit for resetting the charge on the detecting element areutilized.

In the above arrangement, the detecting element takes the form of an ionchamber 1 having its collecting electrode 2 direct coupled to thecontrol grid of a triode 3 having the characteristics of an electrometertube. The plate or anode of the tube 3 is of special design to give itthe characteristics of an electrometer tube, and may take the form of aVictoreen type 5803, VX34, or VX32B, or any other suitable type whichhas the characteristic of drawing low grid current (in the order of 10"A. when out off). The anode of this tube is then coupled through theconventional load resistor 20 to a source of B+ potential, which ispreferably of the i order of 30 volts. The coupling may also includeother conventional types of resistors since the filament or cathode oftube 3 is likewise coupled to the source of B+ potential through aresistor 21. The anode of tube 3 is also coupled through a resistor 22to the control grid of the first section of double triode 6 whichcomprises a part of a conventional Schmitt trigger circuit. In thisarrangement the anode of the first section of double triode 6 is coupledthrough a condenser 23 and a pair of resistors 24 and 25 to thecontrolgrid of the second section of the double triode 6. The cathodes of thefirst and second sections, respectively, of the double triode 6 arejoined together and coupled to ground through a common cathode resistor26. The control grid of the second section of double triode 6 islikewise coupled to ground through a large grid resistor 27 preferablyvalue of voltage divider 34, 35). .Thesecond armature 33 is coupled toone side of the winding of a register or mechanical recorder 8 which isin turn'coupled through the upper wiper arm and one of the contacts ofgang switch to a source of potential.

When the relay 7 is energized, the arnriatures 30,33 are in frontcontact position, and condenser 4 is coupled directly through lead-31,armature 30, front contact ofirelay 7 and voltage divider 34, 35 to B+.'At the same time the second armature 33 e of relay 7 completes acircuit from the source of potential through the upper wiper arm' ofswitch 5, lead 36, front contact of relay 7, armature 33, and lead 37 tothe winding of mechanical register 8. When relay 7 is deenergized botharmatures 30 and 33 are in back contact position and break the circuitsto condenser 4 and mechanical register 8, respectively.

In its operation, a positive voltage from B+ is applied by manuallymoving the start switch 5 so that a circuit is completed from B+ throughvoltage divider 34, 35 contacts of the lower switch of gang switch -5,lead 31 to condenser 4 and then to ground through resistor 32. Thisraises the potential of one set of plates .of the condenser to apredetermined level. However, the other set of condenser plates cannotrise to the same potential since the electrode 2, to which they arecoupled, is directly connected to the grid of the electrometer tube 3.The grid of the'electrometer tube 3 draws grid current and this tube 7acts as a diode clamp which prevents the electrode 2 and leaks olf ofthe first set of condenser plates to. ground through the resistor 32,such action causes the other set of condenser plates to fall inpotential to a point where the electrode 2, coupled thereto, is belowground. This has the effect of biasing the electrometer tube 3 below cutoff.

As an ionizing radiation penetrates the ion chamber 1,

I change from the quiescent state to the triggered'state,

i.e., from the state where the first section of tube 6 is conducting tothe state wherethe second section conducts. Since the plate circuit ofthe second section of triode 6 is connected to B-I- through a largeresistor -29 which serves to slowly charge condenser 38, relay 7 isenergized only momentarily, during the conduction of the second sectionof tube 6, and until condenser 38 is discharged. Since relay 7 is onlymomentarily energized, condenser 4 is recharged to the point where itsnegative potential cuts off tube 3 and returns the Schmitt circuit toits normal state. During this operation the energizing of therelaycompletes a circuit from condenser 4 through line 31, armature 30,front contact of relay 7 voltage'divider 34 and 35 to 3+, and places acharge on condenser 4. At the same time the second armature of relay 7upon reaching front contact position energizes the mechanical register 8and causes it to actuate the counting mechanism to record the count. p

- When the condenser 4 becomes charged, the charge tends to leak off toground through resistor 32; this action causes the control grid ofelectrometer tube 3 to become more negative so as to cut ofi this tube.This causes the potential of the anode to rise to a more positive levelY which is coupled through resistor 22 to the control grid of the firstsection of the double triode 6. This positive level triggers the Schmittcircuit and causes the first section to conduct and the second sectionof tube 6 to cease conducting. When the second section of double triodetube 6 ceases to conduct, the condenser 38 is then per; mitted torecharge. The opening of armature 30 removes-'- the charging currentfromthe condenser 4 and permits it to'remain in its reset position until thecharge thereon is changed by the action of ionization in the radiation"de- 7 tecting instrument. This Iprocessis repeated over'ia'nd lower endof the cylinder receives a Bakelite ring 12 abuts against a shoulder 51formed thereon. Mounted coaxially'of thecylinder and disposed therein isthe collecting electrode 2 of aluminum or other'suitable ma terial. Thelower end of the aluminum electrodez reduced and receives a metal disk.15 of aluminum foil .or, other suitable material. This disk 15 is heldin Place .by. a metal spacer or sleeve 52 which is urged intoclampingrelation with the disk 15 and towards the shoulder53at the upperextremity of the reduced end of the collector The lower end of thereduced portion of collector 21's threaded and carries a nut 54,thereon.Inter-posed be-.' tween the spacer 52 and the nut 54- is aninsulatingelement 11of fluoroethene or other suitable material. Ele meat11 is adapted to abut against a ring shapedelectrode 4 at its outerperiphery. The ring shaped electrode 4 is preferably formed by machiningand maybe of suitable material such as aluminum, and is recess seated inthe insulating ring12. It has a portion Ii -integral therewith whichextends inwardly in overlappingl relation with the disk shaped electrode15 and is adapted to lie along the upper face of the insulatingelemeht'l'l; shaped electrode 4 is held in placein the Bakeliteinsulat-f ing ring 1 2 by means ofa series of screw carried washers 55.The screw carrying washers 55 are threaded intosockets in the insulatingring 12 as indicated at 56. Screw and nuts 57 form the electricalconnectionto the inside of the ion chamber for the acceleratingpotentialfTh'e screws 56 also serve the purpose of maintaining the'ringelectrode 4 in place and supporting the' insulating element 11 inassembled relation. When the electrode. "411s mounted in place in ringshaped end 12- it serves to'clamp disk 13 in place between it and therecessed wall of end 12. This arrangement completes a three platecondenser. consisting of plates 13 and 14 which arein contact electrode4, and plate 15 which is in electrical contact with the collectorelectrode 2. In this arrangement the electrode 4 not only serves as aportion of the condenser but also performs the function of a shield orytrd: ring for the radiation detector. This arrangement disposed within acan 16 of cylindrical shape which comprises cylindrical sidewalls of anydesiredformof metal such as 1 brass or steel with a tin coating andopposed endwalls 58, and 59 of similar material such as brass. The upperend 59 may be held in place by swagging or spinning hbfld edge of thecylinder over the periphery'of end59 and sealing it. The lower end wall58 is removableandis sealed in place by packing ring 60 whichis'adapted'to seat in an annular groove 61 in the outer peripheryioftheend 58. The end 58 is mechanically'joined to 'the eylindrical sidewall'of thecan16 by means of screws -62. which' may be employed foreffecting this connection. After assembly, the ion-chamber is evacuatedand filled with dry air or dry nitrogen to a pressure of one atmosphere.This is accomplished by a copper filling tube 70 in end wall 58. Thefilling tube is threaded and screwed in the end wall and is softsoldered at the joint to produce a seal. For the purpose of mounting theelement 9 in place a plurality of spacers 63 may be employed. Thesespacers serve to position insulating ring 12 above the endwall 58 of thecan. They are a part of a series of mountings which included screws 72which pass through sleeves 63 and openings in plates 71. Screws 72.thread into end 58 and maintain the spacers 63 and plates in assembledrelation. The plates in turn are joined to ring 12 by screws 73 whichpass through additional openings therein and seat in threaded sockets inring 12, as shown in Figure 2.

Disposed within the can and positioned beneath the insulating element 11and the lower end 12 of the radiation detector cylinder is theelectrometer tube 3. This tube may be mechanically supported through thewires or leads 64. These leads are carried to circuits which are locatedexternally of the radiation detecting device by means of a Kovar plate65 through which glass coverings 66 for leads 67 are indicated. TheKovar seal between glass and Kovar plate maintains the isolation of theradiation detector from external atmospheric conditions. The leads 67are then connected into a socket arrangement having a series of prongs68. These prongs are mounted in a conventional base 69 which plugs intoa standard tube socket. The base 69 is then disposed within and clampedto the cylindrical portion 17 whose upper end is screw threaded and fitsinto a socket in the end 58 of the can. The Kovar seal 65 is softsoldered to end wall 58.

From the foregoing, it may be noted that leakage paths have been reducedto a minimum. None of the insulation in the grid and chamber electrodecircuits is exposed to atmospheric conditions which might involvesubjecting either of them to moisture and dirt. Nor is this arrange mentD.C. coupled to any circuit which is exposed to the atmosphere. All leador coupling circuits that come out of the can are of moderately lowimpedance. All high impedance circuitry is enclosed in the can andhermetically sealed. Consequently, it is possible to use smallercondensers and a smaller ion chamber for a given sensitivity ofinstrument than is possible with the conventional instruments of likesensitivity which are available commercially. For comparable sensitivityin an ion chamber which is not hermetically sealed and is not connectedto the electrometer tube in the manner of this invention it would benecessary to provide an ion chamber of the order of several hundredtimes as large as that employed by applicants. This arrangement providesa sensitivity that is physically prohibitive in other known instrumentsof comparable size.

Having thus described our invention, we claim:

1. A radiation detector for an integration system comprising a housing,a closure for the housing, means for hermetically sealing the closure tothe housing, spaced 'inner and outer electrodes concentricallypositioned in the housing and defining a particle ionization regiontherebetween, said inner electrode serving as a collector electrode, anda condenser positioned within the housing and coupled to the innerelectrode to serve the dual function of a capacitor and a guard ring.

2. A radiation detector as described in claim 1 wherein an electrometertube is positioned within the enclosure and has its control griddirectly coupled to the inner electrode to minimize leakage.

3. A radiation detector for an integration system comprising a housing,means for hermetically sealing the housing, spaced inner and outerelectrodes concentrically positioned within the housing, said innerelectrode acting as a collector electrode, and a condenser having aplate directly mounted on and coupled to the inner electrode, saidcondenser having a second plate mounted on and insulated from the outerelectrode and positioned between said particle ionization region andsaid first plate.

4. A radiation detector as described in claim 3 wherein an electrometertube is positioned within the enclosure and has its control griddirectly coupled to the inner electrode to minimize leakage.

References Cited in the file of this patent UNITED STATES PATENTS

